The present invention relates generally to sequence controllers of image forming apparatuses which are used as terminals of a host computer, and more particularly to a sequence controller which is set up between a host computer and an image forming apparatus such as a laser printer and stores time-sequential commands from the host computer. The commands include complex path selection commands and a print start command, and the sequence controller can change the stored commands if needed.
A conventional printer is designed to prohibit acceptance of a next print start command during a predetermined time while the printer carries out a print operation responsive to a previous print start command.
A description will be given of an operation of the conventional printer with reference to FIGS. 1 and 2. First, as shown in FIG. 1, a recording paper is supplied from one of an upper paper supply tray 1, a lower paper supply tray 2 and a paper supply tray 3 having a large capacity 3 by a corresponding one of paper supply rollers 4, 5 and 6. Resist rollers 7 control a transport timing of the recording paper and transports the recording paper to a photoconductive drum 8. The photoconductive drum 8 rotates clockwise and a surface thereof is charged by a charger 9. A laser writing unit 10 modulates an image signal into a light beam. A light beam L from the laser writing unit 10 is irradiated on the surface of the photoconductive drum 8 thereby forming an electrostatic image on the surface of the photoconductive drum 8.
The electrostatic image is visualized into a toner image by a developer 11 which forms a toner layer. A transfer charger 12 transfers the toner image onto the recording paper which is transported to the photoconductive drum 8. The recording paper is further transported to a fixing unit 14 by a transport belt 13, and the toner image is fixed on the recording paper. After passing the fixing unit 14, the recording paper is ejected to one of a lower eject tray 17, an upper eject tray 18, a side eject tray 19 and a duplex unit 20 via path selectors 15 and 16. After the transfer of the toner image, the photoconductive drum 8 is cleaned by removing residual toner by a cleaning unit 21.
The printer is coupled to a sequence controller 23 and the sequence controller 23 is coupled to an image data processor 22. The image data processor 22 is coupled, on the other hand, to a host system 24 such as a computer and a word processor. The image data processor 22 converts print data codes into laser modulating data. The sequence controller 23 controls the operation sequence of the printer. The image data processor 22 and the sequence controller 23 are located at an upper part of a main printer body.
FIG. 2 generally shows the image data processor 22 and the sequence controller 23. In FIG. 2, the image data processor 22 converts image data received from the host system 24 into the laser modulating data and selection signals for selecting paper supply paths, paper eject paths and print modes such as a one-sided mode, a two-sided mode. The sequence controller 23 controls the printing operation of the printer 25 in response to a print request signal from the image data processor 22.
Said sequence controller 23 comprises a serial input and output interface 230, a parallel input and output interface 231, a central processing unit (CPU) 232, a read only memory (ROM) 233, and a random access memory (RAM) 234 which are coupled to each other via a bus 235. The serial input and output interface 230 is coupled to the image data processor 22. The parallel input and output interface 231 is coupled to the printer 25. The CPU 232 carries out a general control and controls the entire operation of the sequence controller 23. The ROM 233 stores programs for controlling the CPU 232. The laser modulating data and the selection signals from the image data processor 22 are stored in the RAM 234.
The printer 25 further comprises an operation and display part (not shown) including keys for inputting various instructions and a display for displaying various messages.
A description will now be given on the operation of the system shown in FIG. 2. When all print data related to a print start command from the host system 24 are received at the image data processor 24, the image data processor 24 supplies to the sequence controller 23 the selection signals for selecting paper supply paths, paper eject paths, print modes and the like. And when the sequence controller 23 receives a print start signal, the CPU 232 starts a print sequence control.
As for command transmission, command codes are transmitted from the image data processor 22 to the sequence controller 23 by a serial communication method, while the commands are transmitted from the sequence controller 23 to various parts of the printer 25 by a parallel communication method.
The printer 25 is designed to prohibit acceptance of a next print start command during a predetermined time while the printer carries out a print operation responsive to a previous print start command. Consequently, the image data processor 22 must postpone generation of the print start command until the printer 25 is ready for the next print operation even if all the necessary print data are received at the image data processor 24. This is a barrier to improvement of the processing speed.
To solve this problem, it is conceivable to make the sequence controller 23 accept the next print start command before the printer 25 assumes a ready state in which the next print operation can be carried out, and successively store the related selection information such as the paper supply and eject paths in the RAM 234. In this case, the printer 25 carries out a print operation in accordance with the selection information stored in the RAM 234 whenever the printer 25 assumes the ready state. But even in this case, when a problem occurs in the selected paper supply and eject paths, such as the case where paper runs out when the print operation is to be carried out, the CPU 232 stops the print operation in response to a detection signal from the printer 25. In order to restore the printer to the ready state, an operator manually removes the problem. In above described case, the operator must carry out the troublesome operation of setting a sufficient supply of paper in the concerned paper supply tray.